Due to the advancement of related semiconductor process technology and demand for diverse application of electronic products, the trend is toward miniaturization and high efficiency of semiconductor devices. As a result, the semiconductor industry is confronted with an issue, that is, heat dissipation of semiconductor devices, especially when it comes to application of high-frequency and high-power devices.
Take light-emitting diode (LED) devices as an example, although LEDs are highly efficient because they save power, but their weakness in heat dissipation hinders their application in high-power fields, such as headlights. The efficiency of light emission of LEDs depends on temperature. If heat emitted from LEDs is not dissipated away from the LED devices, the efficiency of light emission of the LEDs will deteriorate to the detriment of operation stability and service life. The unstable operation of LED devices not only poses a threat to personal safety and properties, such as headlights, but also seriously impedes the LED devices' market penetration.
Take GaN-based high electron mobility transistors (HEMT), with GaN semiconductor being characterized advantageously by high electron saturation velocity, high thermal stability, and high breakdown voltage, AlGaN/GaN HEMT displays high output power and thus is widely applied to microwave power amplifiers and transformers. However, waste heat generated as a result of high-power operation reduces the efficiency of devices greatly.
To reduce the aforesaid unfavorable effect of waste heat on the aforesaid devices, heat is usually removed away from the devices by heat dissipation design and management. The related prior art involves transferring a device onto a substrate with a high thermal conductivity coefficient. However, the transfer requires removing the device from the original epitaxial substrate by laser lift-off and bonding the device to the substrate with a high thermal conductivity coefficient by a wafer bonding technique. As a result, the transfer not only increases the manufacturing costs and complexity, but also compromises the yield. The related prior art also involves carrying out epitaxial production of devices (such as GaN LED or HEMT) with a monocrystalline substrate (such as a monocrystalline AlN (aluminium nitride) substrate) with a high thermal conductivity coefficient, as the high thermal conductivity of AlN is beneficial to heat dissipation and elimination of the need for device transfer. However, monocrystalline AlN substrates are difficult to obtain and expensive and thus render mass production infeasible.
Research on production of GaN materials by polycrystalline AlN substrates shows that polycrystalline AlN substrates are not uniform in rigidity and thus cannot attain nanoscale surface roughness simply by any polishing techniques, and in consequence the polycrystalline AlN substrates have rough surfaces to the detriment of subsequent GaN epitaxial growth. In addition to flattening, the related prior art is confronted with another problem, that is, a buffer layer is made of a material with c-axis preferred orientation, such as ZnO or AlN, to function as a pre-growth layer with correct c-axis upper lattices on which a GaN wafer grows. However, coalescence growth required for thin-film formation cannot occur in the same in-plane direction as the lattices, because polycrystalline AlN substrates are confronted with a problem, that is, random lattice surfaces.
As a result, the industrial sector needs a method of manufacturing an epitaxiable heat-dissipating substrate so that the epitaxiable heat-dissipating substrate has a high degree of flatness and high quality, and thus is suitable for manufacturing semiconductor devices anticipated by the industrial sector.